1. Field of the Invention
This invention relates to a method and apparatus for measuring the force applied to a seatbelt. Specifically, a belt force sensor is mounted on a plate positioned between a fixed mounting member and a movable mounting member to isolate the sensor from movements due to non-axial loads.
2. Related Art
Most vehicles include airbags and seatbelt restraint systems that work together to protect the driver and passengers from experiencing serious injuries due to high-speed collisions. It is important to control the deployment force of the airbags based on the size of the driver or the passenger. When an adult is seated on the vehicle seat, the airbag should be deployed in a normal manner. If there is an infant seat or small adult/child secured to the vehicle seat then the airbag should not be deployed or should be deployed at a significantly lower deployment force. One way to control the airbag deployment is to monitor the weight of the seat occupant.
Current systems for measuring the weight of a seat occupant are complex and expensive. One type of system uses pressure sensitive foil mats mounted within the seat bottom foam. Another system uses sensors placed at a plurality of locations within the seat bottom. The combined output from the mats or the sensors is used to determine the weight of the seat occupant. The accuracy of the weight measurements from these types of sensor systems can be compromised due to additional seat forces resulting from the occupant being secured to the seat with a seatbelt.
For example, weight sensor systems can have difficulty identifying an adult, a child, or a car seat when the seatbelt is being used. When a child seat is secured to a seat with a seatbelt, an excess force acts on the sensors mounted within the rear portion of the seat bottom, which interferes with accurate weight sensing. Over tightening of the seatbelt to securely hold the child seat in place, pulls the child seat down against the rear part of the seat bottom, causing the excessive force measured by the sensors. Due to this effect, the current weight sensing systems have difficulty in discerning between an adult belted to a seat and a child seat secured to the seat with a seatbelt.
In order to address this problem, sensors have been incorporated into the seatbelt to measure the tension force applied to the seatbelt as passengers or a child seat is secured to the seat. High seatbelt tension forces indicate that a child seat is secured to the seat. One current belt force sensor is mounted to a metal plate anchored at one end to a vehicle structure and mounted at an opposite end to a seatbelt latching member. The sensor measures the strain applied to the plate to determine the tension force on the seatbelt. One disadvantage with this system is that it is difficult to get accurate strain measurements for the lower operating ranges. The metal plate, as a direct connecting member between the seatbelt latching portion and the vehicle structure, must be thick enough to withstand overloading on the seatbelt caused by high vehicle impact/collision forces. However, because the metal plate is so thick, there is not enough strain movement of the plate at the normal operating ranges of the sensor to accurately measure seatbelt tension.
Another disadvantage with this type of sensor configuration is that non-axial loading on the metal plate can affect the accuracy of the tension force measurement. Preferably, as the seatbelt is tightened, a linear or axial force is applied to one end of the metal plate and the sensor measures the strain caused by this axial loading. However, if a non-axial load is applied to the plate by pulling on one end of the metal plate at an angle, the sensor may provide an inaccurate reading.
Thus, it is desirable to have a system for measuring seatbelt forces to determine whether a child seat or an adult is secured to the seat that utilizes a sensor that is isolated from non-axial movements, and which can provide a high strain level for the normal working range of the sensor. The system should also work with traditional seat occupant weight sensing systems, provide increased accuracy, be easy to install, as well as overcoming any other of the above referenced deficiencies with prior art systems.
A seatbelt sensor system includes a sensor that is mounted to a floating or isolated member positioned between a fixed member and a movable member. The fixed member is preferably mounted to a vehicle structure and the movable member is mounted for movement relative to the fixed member in response to a seatbelt input force. The floating member is preferably a thin resilient member that responds to movement of the movable member to provide high strain levels for a normal working range of the sensor. An overload stop prevents the separation of the movable member from the fixed member when a predetermined seatbelt input force limit is exceeded.
In a disclosed embodiment of this invention, the fixed member and the movable member are significantly thicker than the resilient member that supports the sensor. The movable and fixed members work together to define the overload stop. The fixed member includes a transversely extending portion that extends through the resilient member and the movable member in a first direction. The movable member includes a transversely extending portion that extends through the resilient member and the fixed member in a second direction, opposite from the first direction. The openings in the fixed and movable members through which the transversely extending portions extend are greater in size than the openings in the resilient member through which the transversely extending portions extend. This allows the movable member to provide input to the resilient member while also providing an overload stop between the fixed and movable members.
A method for measuring seatbelt forces includes the following steps. One structural member is fixed to a vehicle structure. Another structural member is mounted for movement relative to the first member in response to a seatbelt input force. Another member, which supports a sensor, is positioned between the first and second structural members. A seatbelt force, applied to this middle member by the moving structural member, is measured by the sensor. Further, separation of the moving structural member from the fixed member is prevented if a predetermined seatbelt force limit is exceeded.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.